It is known to perform surface treatment on metal parts in order to obtain properties in terms of friction and lubrication that are needed for their use, where the treatment is conventionally electrolytic chromium plating. An electrolytic chromium plating makes it possible to obtain a hard chromium coating and it is in very widespread use in various fields such as the field of aviation, because of its excellent properties in terms of friction, resistance to wear, and providing protection against corrosion. Electrolytic chromium plating is generally finished off by rectification so as to guarantee that the coating is of thickness that is uniform and presents a surface state corresponding to surface roughness (Ra) that is less than 0.2 micrometers (μm). The success of the above technique can be explained by the fact that the characteristics obtained after such treatment steps include firstly excellent friction strength because of good resistance to wear associated with a perfect surface state, and secondly excellent lubrication in the presence of fluids due to the microcracking effect that is inherent to hard chromium and that provides a retention zone.
Nevertheless, hard chromium plating is performed in an electrolytic cell in the presence of chromic acid based on hexavalent chromium (Cr6+), which is harmful to the environment and to human beings. That substance is classified as being CMR (carcinogenic, mutagenic, and harmful for reproduction). In addition, like numerous electrolytic methods, that substance embrittles steels because of hydrogen diffusion, and it requires operating precautions to be taken in order to avoid burn marks in the underlying steel after rectification, where such burn marks give rise to irreversible degradation of the treated metal part.